Thermionic cathodes



THERMIONIC CATHODES 2 Sheets-Sheet 1 Filled July 28. 1954 Inventor: A.H. W. BECK A.B.CUTT|NG- A D. BRISBANE- 6. KING Attorney United States Patent THERMIONIC CATHODES Arnold Hugh William Beck, Alan Butler Cutting, Alan Douglas Brisbane, and George King, London, England, assignors to International Standard Electric Corporation, New York, N .Y.

Application July 28, 1954, Serial No. 446,206

Claims priority, application Great Britain August 14, 1953 7 Claims. (Cl. 313-346) The present invention relates to thermionic cathodes.

For the purpose of providing a cathode having long life and copious thermionic emission, it has been proposed to construct a cathode from sintered metal, such as tungsten, forming a housing for a cathode heater and a reservoir in which materials of high thermionic emissivity are contained. With barium oxide as the emissive material, it is believed that atoms of barium migrate through the sintered cathode wall during operation and liberate electrons from the cathode surface. Generally speaking the production of dispenser type cathodes, as they may be called, has not been very successful in the past thirty or more years during which they have been investigated. This is due to a variety of chemico-physical factors, such as pore size, critical proportions of the materials to be used, the problem of finding a suitable reducing agent for a given mixture of base material and emissive material, and also the important factor, in some types, of satisfactory adhesion to a base support.

According to the present invention there is provided a dispenser type cathode in which the active cathode material is formed from a pressed and sintered mass of mixed powders of the metals nickel or cobalt and thermionically emissive alkaline earth material.

The compacted thermionically emissive mass may be used as a surface layer, having any desired thickness, applied to a sintered nickel base; or be used to form a plug, held in a suitable sleeve, or even as a directly heated self supporting cathode, the compacted material being sintered during manufacture.

f Embodiments of the invention will be described with Patented Nov. 10, 1959 2 after processing. At 3 ampsf/cmF, cathodes having lives of over 2500 hours and at 1 amp./cm. cathodes having lives of over 4000 hours have been obtained. 7

The important features of the cathode ofFig. 1 which enable its successful production are believed to be, be sides the processing details, the use of zirconium hydride as a reducing agent, carbon having been found to be unsuitable with thicknesses of emissive layers exceeding 0.002 inch, and the presence of the sintered nickel layer 4, which enables satisfactory adhesion of the active layer 5 onto the base 3 of the cathode cup 1. t

The manufacture of the cathode prior to activation and'final processing is as follows:

A mixture of 69% nickel powder, 30% mixed barium and strontium carbonates, and 1% of a zirconium reducing agent is prepared. The nickel powder is purchased in the form of carbonyl nickel powder having a particle size of 1-5 microns diameter, and, before use, is stoved in vacuum at 400 C. The barium-strontium carbonate mixture is a standard double carbonate powder such as commonly used in the manufacture; of thermionic cathreference to the accompanying diagrammatic drawings in Fig. 3 shows a cross-section through a plug type of cathode having layers of different composition;

Fig. 4 shows curves relating to emission current and temperature for cathodes of the invention and previously known types of cathode; and

Fig. 5 shows a modification of the cathode of Fig.2.

In Fig. 1 of the accompanying drawings a cup 1 of nickel, molybdenum, tungsten, tantalum, nickel-iron alloy or other suitable metal, of the shape normally used in the manufacture of a planar cathode for an electron gun, encloses a conventional type of heater 2. The end surface 3 of the cup carries a coating 4 of nickel powder sintered toit, and an outer layer 5 of what we shall refer to as active material pressed on to the layer 4 and finally sintered. This layer of active material 5, whose preparation will be described more fully below, is formed initially of an intimate mixture of finely divided nickel, alkaline earth carbonate or carbonates and zirconium hydride, a

odes. The zirconium is in the form known commercially as zirconium hydride, whichis really a solution of hydrogen in zirconium. 30 grams of the nickel, carbonate and zirconium hydride mixture is ball-milled in 50 millilitres of amylacetate for 30 minutes, and is then filtered and dried in air at 110 C. The resultant powder is then stored for further use.

The surface 3 of the cathode cup of Fig. 1 is covered with a thin layer of nickel by spraying or brushing a suspension of suitable nickel powder onto the surface. The same carbonyl nickel powder mentioned above is used, suspended in amylacetate. This nickel is sintered onto the surface by stoving in vacuum or hydrogen at 1000 C. to form the layer 4, whose purpose, as mentioned'above, is primarily to improve the tenacity of the surface layer 5 onto the cathode cup. The layer 4 should essentially provide a rough surface, and from this point of view a thin layer, such as produced by brushing, is to be preferred. On to this surface is brushed or sprayed emissive powder, prepared as described above, suspended in amylaceta'te. The cathode is completely dried at 110 C. and is then pressed against a polished plate, an applied pressure 'of between 20 to 100 tons per square inch being satisfactory. The final thickness of the emissive layer is not critical and may be from 0.001 inch to more than 0.004 inch. If it is desired to store the cathode for any length of time it is advisable to give it a protective coating such as may be obtained by dipping it in a solution of methyl methacrylate plastic dissolved in acetone. A plastic known commercially as Diakon has been successfully used. At this stage in the preparation of the cathode the active'layer 5 is of metallic appearance and may be machined, although it is possible, and in some circumstances it may be desirable, to postpone lathe or like cutting operations until after activation.

The activation process of the layer type of cathode shown in Fig. 1 is essentially the same as that required for the plug type of cathode shown in Fig. 2, which will now be described, the details of the activation process being discussed subsequently.

copious continuous emission of 3 amperes or more per 0 square centimetre of active-surface area being obtained inch; After pressingjthe formedcathode'is'eim'acted' from the steel ring and, if not required for immediate use, is coated with Diakon.

Although it is not strictly essential, it has been found advisable, in some cases, to incorporate a solid metal backing plate for the plug 7, such as is indicated at 8 in Fig. 2. This backing plate not only provides additional mechanical support for the plug 7, but also prevents evaporation of thermionically emissive material into the heater space.

When required for activation, cathodes, formed as described above in connection with Figs. 1 or 2, and provided with heaters 2, are assembled into envelopes either together with the other electrodes with which they will eventually cooperate, or each with a convenient anode to form diodes from which the cathodes will be extracted after activation. These assembled tubes are mounted on a pumping station in normal manner, and are baked for one hour, during which, at a temperature between 350 and 400 C., the Diakon, if present, depolymerises With-considerable evolution of gas.

After baking, the anode and any other electrodes are out-gassed in normal manner by heating to a higher temperature and are maintained hot during the subsequent break-down of the carbonates into oxides, which is carried out in the normal way by heating at a considerably higher temperature until carbon dioxide ceases to be evolved. After the reduction of the carbonates to oxides is complete, the cathode temperature is lowered somewhat and electronic emission is drawn from the cathode by applying a suitable voltage to the anode and any other electrodes. In the initial stages of activation, if too high a current density is drawn from the cathode it readily be comes poisoned. At low current densities the thermionic emission increases with time at a rate depending upon the emission current density. The cathode is therefore activated by drawing the maximum current, depending on the state of activation of the cathode and the temperature, which avoids the above mentioned poisoning effect until the desired emission density is obtained. This activation does not result in any appreciable amount of gas being evolved'and may be performed, if desired, after the valve is sealed off, provided the electrode assembly includes a getter which is fired before activation.

During the activation process, the active material, whether itbe a surface layer such as 5, Fig. 1, or a plug 7, Fig. 2, or each of the layers of a composite type of cathode to be described later, becomes sintered. After activation the cathodes are quite stable, and may be exposed to the normal atmosphere without permanent poisoning. Hence, if desired, the cathodes may be activated, as suggested above, in temporary diode structures from which they are subsequently recovered. The activation process is effective for substantially the whole thickness of the active material, and the latter may be machined or ground, if desired, at this stage. Since the surface is now sintered, it may be machined more readily than before activation, and without permanent deterioration. Nevertheless, as mentioned previously, the active material may be machined before the activation process, if desired, although not such a high surface polish can be obtained as when the machining is performed after activation. The activated and sintered material can also be welded.

After any such welding or machining operations have been performed, the cathode is assembled into the discharge tube for which it is finally required, the tube then being outgassed in normal manner. Only a short reactivation process is required.

Investigations of experimental cathodes undergoinglife testsindicatethat there is no undue evaporation of barium from the cathode surface. Nevertheless, the difiusion rate through, andhence the evaporation rate of barium from the surface of the cathode may be decreased, if required, by varying the percentage of nickel and carbonate near the emitting surface of the active material of the cathodes, so altering the porosity of the sintered mass. For a homogeneous plug, the proportions of nickel, carbonate mixture and reducing agent may be varied considerably.

A plug type of cathode in which the porosity of the sintered material is varied so as to be comparatively small at the cathode surface, and larger in the interior, is shown in Fig. 3, in which a composite plug 9 is inserted in a cylinder 6 of nickel or other refractory metal. The plug is shown formed of four layers of active material. The layer 10 contains 40% of alkaline earth carbonate in the original powder mixture, the layer 11 20%, and the layer 12 10% of carbonate. The pore size will be larger with the larger percentage of carbonate. The under surface of the plug comprises a layer 13 of nickel powder without any admixed thermionically emissive material. Each of these layers may vary in thickness from, say, 0.010 inch to 0.020 inch. Inpreparing the cathode of Fig. 3 the various layers may be made one at a time, for ex ample, by first fitting the sleeve 6 with a suitable backing plate, putting in nickel powder to form the layer 13, then compressing this layer, then adding the next layer of active material, pressing this, and so on until the plug is formed, after which it is activated as described. Alternatively, the loose powders can be inserted one at a time and the plug compacted in a single operation. The provision of the layer 13 of nickel powder substantially prevents emission into the cathode heater space. If desired, it may be replaced by a solid metal backing plate, such as the plate 8 of Fig. 2.

The above embodiments have been described using nickel powder in the active material. We have found that, if desired cobalt may be substituted for powdered nickel.

The cathodes of the present invention provide, at a given operating temperature, values of thermionic emission density considerably greater than those published-for other types of dispenser cathode, or, to put it another way, they give the same emission as known types but at a lower temperature. Life tests reveal, as mentioned above, that very long lives, even at remarkably high emission densities, are achieved. An indication of the relationship between emission and cathode temperature for cathodes of the present invention and some other known types of cathode is shown in Fig. 4, in amperes per square centimetre, is plotted as ordinate, and true cathode temperatures in degrees Centigrade (as opposed to optical pyrometer readings giving an apparent brightness temperature, which must be corrected for the thermal emissivity of the cathode) are plotted as abscissae. Curve A shows the variation of pulsed emission obtainable at various temperatures with known types of oxide coated cathode, a maximum temperature of about 900 C. being indicated for a useful life of 1000 hours.

Curve B refers to the pulsed emission obtainable from a type of dispenser cathode involving diffusion through porous tungsten, and is based on published figures; a much higher maximum temperature of some 1200 C. may be used. The shaded area'C gives the approximate range of cathodes of the present invention based upon tests up to the present time. It will be seen that the operating temperatures are intermediate between those of curves A and B. No indication of any limiting maximum temperature has yetbeen obtained below the melting point of nickel. The limitations in this respect have been due to difficulty in cooling the anodes'of experimental diodes in which the cathodes have been incorporated.

To obtain a life of more than 3000 hours, it is generally accepted that the continuous emission from the ordinary oxide coated cathode must be limited to 0.5 amp./cm. The continuous emission-temperature curve for the conventional oxide cathode is shown at D. The curve extends up to 1 amp/cm. at which the life of the cathode is normally considerably less than 1000 hours. Curve E relates to cathodes of the present invention giving continuous emission. The upper limit of the curve is not yet known, nor is the maximum temperature for active material 1 a reasonable life; on the other hand, while early samples of cathodes to which the curve relates have had lives of 2500 hours when operated continuously at 3 amps./ cm. later samples show even longer lives without any sign of being over-run.

In most cathodes so far made according to the present power, and thus the cathode mounting, including the supporting sleeve or cylinder, accounts for a very'large proportion of the heater power required. Even so, the heater powers required have been within the range normally accepted for oxide coated cathodes. By way of example, a cathode of the type illustrated in Fig. 2 had a nickel cylinder 6 of length 0.236 inch, 0.180 inch outer diameter and 0.158 inch internal diameter, accommodating a plug 7, 0.025 inch'thick. For this cathode, for an emission current of 0.4 ampere, a heater power of 7.1 watts was required. For comparison, of about e watts would be required to raise a nickel cylinder of the same dimensions to the same temperature, if conduction cooling by the necessary supports is completely ignored.

It will be evident that cathodes according to the present invention, owing to their high emission capabilities, will find immediate applcation in the field of cathode ray tubes, high voltage rectifiers and beam tubes for high frequency use.

The cathode described in the present specification have all had a planar emitting surface. Surfaces of a shape such as required for some types of electron guns may readily be made by employing dies of suitable configuration in the pressing of the active material of Furthermore, in certain applications, a rod or tube of the active material, may be usecias a directly or indirectly heated cathode. The material requires sintering be fore assembly into a discharge tube with other electrodes, i.e. sintering must be done before activation and not durprocess, connections to the cathode being made by welding. By way of experiment, a directly heated cathode was made up in the form of a isfactory emission was obtained.

As has been mentioned previously, and exemplified in Fig. 3, the proportion of electron emissive material to nickel in the mixture from which cathodes of the present invention are made does not appear to be critical.

With a view to obtaining a lower work function at the cathode surface than obtains results have been achieved of a metal of lower work function than nickel or cobalt combined with a plug or layer of active material such as already described. Thus in Fig. a cathode otherwise similar to that of Fig. 2 is shown but having an outer layer 14 of sintered metal of lower work function such as tantalum, tungsten, zirconium, titanium, thorium or silicon. In the manufacture of this invention the layer 14 is first formed and sintered separately. The layer 14, the active material of the plug 7 in the initial powder form and the backing plate 8 are ina power troduced into the sleeve 6 and are compressed together The cathode is then processed as has previously been de scribed. In a multilayer cathode such as described witl reference to Fig. 3, an outer layer such as 14, Fig. 5 may be included or may replace the upper layer 1'2 containing the 10% carbonate mixture. hile the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only of way of example and not as a limitation on the scope of the invention.

What we claim is;

1. An indirectly heated cathode comprising a sintered of mixed powders of metallic nickel, thermionically emissive alkaline earth material, and a a metal support member to which said united, said support including a heater compartment, the said mixed powders containing from 10% to 40% by weight of alkaline earth carbonate, a proportion not exceeding 1.5% of said reducing agent and the remainder pure nickel.

2. The cathode of claim 1, wherein the nickel powder has a particle size of between 1 and 5 microns, and the ratio of nickel to alkaline earth material is in the range of 2:1 by weight.

3. The cathode of claim 1, further comprising "an intermediate nickel layer between said mass and said support 4. The cathode of claim 1, wherein the mass is layered and the porosity is varied throughout the mass by varying the proportions of alkaline earth metal in the different layers.

5. The cathode of claim 4-, wherein the least proportions of alkaline earth is in the layer contiguous to said refractory metal support.

6. The cathode of claim 1, wherein the compacted mass is a body contained at one end of the heater compartment and a layer of nickel provides an end plate for said compartment.

7. The cathode of claim 4, wherein the layers comprise respectively percentage compositions of alkaline earth material in the range of 10 to 40%.

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